The present invention relates to a method for controlling a stepless automatic transmission of a vehicle and an apparatus therefor. More precisely, the apparatus and the method automatically controls the transmission ratio according to the change with respect to time of throttle aperture and driving condition of the vehicle.
Prior technology for automatically controlling a transmission ratio of a stepless transmission will be explained, for comparison, as follows.
A typical stepless transmission often used in a two-wheeled-vehicle, U.S. Pat. No. 4,543,077 for example, comprises a pair of pulleys connected to a drive shaft and a driven shaft, respectively. An endless belt is wound around the pair of pulleys so as to transmit a drive force from a drive pulley connected to the drive shaft to a driven pulley connected to the driven shaft. Each pulley has a pair of convex disks facing to each other so that the distance between them increases radially from inside to outside. The endless belt is disposed between the pair of disks. The distance between the facing disks is changeable. As the distance between the disks increases, the endless belt comes radially inwards. As the distance between the disks decreases, the endless belt goes radially outwards. Thus, the effective diameter of the pulley, that is the diameter by which the endless belt is wound around the pulley, can be varied by varying the distance between the disks and, consequently, transmission ratio can be varied.
Transmission ratio of the above-mentioned stepless transmission is controlled, according to a prior technology, as follows.
According to a prior technology, throttle aperture is controlled by a driver, and the objective engine speed and the transmission ratio are determined automatically by a prescribed procedure, as follows. First, an objective engine speed is determined according to a prescribed function which is generally an univocal and monotonously increasing function of the throttle aperture. The function is determined so as to give, for example, such an engine speed as provides a most economic driving of the vehicle, extracts a highest power output of the engine, etc. at every throttle aperture. The prescribed function is stored in memory means. Therefore, given the throttle aperture, the objective engine speed can be calculated automatically. Second, an objective transmission ratio is calculated and the transmission is adjusted to the objective ratio so that an actual engine speed coincides with the objective engine speed under the current speed of the vehicle.
According to the prior technology, the only information determinative of the objective engine speed, and consequently the transmission ratio, is the throttle aperture. In other words, whether the throttle is opened quickly or not does not affect the objective transmission ratio. Therefore, the prior technology does not provide a delicate control of a vehicle and sometimes caused inconveniences which may spoil the driving feeling of the vehicle.
For example, when a driver wants to accelerate the vehicle quickly, he has to open the throttle widely so that the transmission is set to a higher ratio. But this operation inevitably leads to an intensified acceleration, which may not be required always, corresponding to a wide aperture of the throttle. On the contrary, if it were for a conventionally geared transmission, the driver can shift up quickly and obtain a desired acceleration.
Another example is when an engine brake is needed. In order to obtain an effective engine brake according to the prior technology, it is necessary to close the throttle largely. Then the transmission is set to a high transmission ratio and an engine brake is obtained But, as mentioned above, this operation inevitably leads to an intensified engine braking of the vehicle which may not be always required.
In other words, intimate control of the vehicle such as a subtle acceleration and engine braking with limited intensity can not be realized by virtue of the prior technology. cl SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a method for controlling a stepless transmission for realizing an improved driving feeling. For the object, a change with respect to time of the throttle aperture is taken into account in determining the objective engine speed and the transmission ratio.
According to a first aspect of the present invention, there is provided an apparatus for controlling a stepless automatic transmission of a vehicle which comprises (a) a throttle sensor for sensing a change with respect to time of a throttle aperture, (b) a first objective engine speed allocator for allocating an objective engine speed in the condition that the change with respect to time of the throttle aperture is within a prescribed range, (c) a second objective engine speed allocator for allocating an objective engine speed in a condition that the throttle aperture is opened more rapidly than a first prescribed value, (d) a third objective engine speed allocator for allocating an objective engine speed in a condition that the throttle aperture is closed more rapidly than a second prescribed value, and (e) a transmission controller for controlling a transmission ratio of the stepless automatic transmission so that an actual engine speed coincides with the objective engine speed.
According to a second aspect of the present invention, there is provided a method for controlling a stepless automatic transmission of a vehicle, the method comprising the steps of (a) sensing a change with respect to time of a throttle aperture, (b) allocating an objective engine speed according to the change with respect to time of the throttle aperture, and (c) controlling a transmission ratio of the stepless automatic transmission so that an actual engine speed coincides with the objective engine speed.